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The Season 10 finale of Doctor Who hits the ground running with a bravura climax, pulling out all of the stops and refusing to pull its punches as the Doctor and Bill’s 12-part journey comes to an end. Viewers who skipped last week’s episode “World Enough and Time” should absolutely jump back and watch it before approaching this second part.

The Doctor (Peter Capaldi) is trapped on a massive colony ship caught in the gravitational pull of a black hole. Bill (Pearl Mackie) has been irreversibly transformed into a Cyberman, joining a growing army of them that are filling the ship. Two incarnations of the Doctor’s arch-nemesis, the Master (John Simm) and Missy (Michelle Gomez) appear to have joined forces. Above everything else, the spectre of regeneration looms in Capaldi’s penultimate episode in the role. After this, there is only the annual Christmas episode to go.

Series producer and writer Steven Moffat has a long-running tendency to surprise with his two-part storylines. Often the first half will set up a particular expectation, and the second will then unexpectedly subvert that set-up with something quite different in content and tone. While last week’s episode spent much of its time with the Doctor on the flight deck of a spaceship and Bill living years of her life in a dystopian hospital, “The Doctor Falls” primarily focuses on the Doctor defending a farmhouse of children from a Cyberman army. Far from being a disappointment, this relatively simple set-up frees much of the episode to focus on its lead characters. While there is plenty of action, the episode’s real strength lies in the protagonists: where have they come from, and how the events of “The Doctor Falls” changes them forever.

For Bill it means coming to terms with her new body, and the knowledge that she could succumb to new programming at any time; programming that would effectively erase her identity for good. It also means settling with the Doctor that his promise to keep her safe has been irrevocably broken, and that there is not an easy fix on the horizon to make everything okay again. Pearl Mackie has been one of the greatest strengths of this season. Her character has been well developed, and her upbeat and naturalistic performance has made Bill one of the most appealing companion characters in Doctor Who’s current form. If this is the end for Bill (and never say never), it is a shame we only got her for the one year. She had so much potential to feature for so much longer.

For Missy the episode means finally making a choice between being a good person or a bad one. This has been the first time Doctor Who has undertaken a story with multiple versions of the Master, and it pulls it off brilliantly. John Simm plays a darker and less playful version of the character than he did back in David Tennant’s days, and it is a take that suits him much better. The interplay between Simm and Gomez does not overpower the rest of the episode but is – one or two poorly chosen jokes aside – hugely satisfying.

Nardole (Matt Lucas) gets an exit from the series as well. He has been a somewhat difficult character throughout the season, working well in his early minor appearances and then grating the more material he is given. Here he works surprisingly well, due in part to his personality been a little understated by comparison the rest of the year.

Finally there is the Doctor himself. He has failed his companion, he has failed to convert Missy to becoming one of the good guys, and minutes into the episode he is mortally wounded and visibly running on fumes. Capaldi gives a towering performance, filled with guilt, sadness, empathy and rage. It is hugely emotional. In the episode’s climax, as he darts between the trees blowing up Cybermen with his sonic screwdriver and shouting out the names of all of the planets where he has previously defeated them, there is a profound combination of joy and sorrow.

‘Without hope, without witness, without reward,’ the Doctor says, explaining to the Masters how it matters just to be good, and to try and save the world whether you succeed or not, and whether you live or die. Nobody has hope in this episode. Critically, no lead character finds out the fate of anybody else. Nobody ultimately gets what they want. Doctor Who has never had a season finale this devastating, or as good.